1. Field of the Invention
The instant invention is related to the automatic control of industrial processes. In a typical installation a process variable monitor or transmitter, such as a temperature probe, is used to generate a process value signal or PVS related to the present value of a process variable, such as temperature. A setpoint generator provides a set-point signal or SPS which is equal to or related to the desired value of the process variable signal as determined by process requirements. An automatic process controller is used to generate a process control signal or PCS which is used to control a final operator intimately related to the process variable such as a flow valve operator. In this manner the temperature of a liquid in a boiler, for example, may be controlled automatically by varying fluid flow through a valve.
This invention further relates to multi-mode process controllers of the type having automatic, manual and direct digital control modes of operation.
In the automatic control mode of operation a control circuit is used to generate an automatic control signal or ACS by applying a control algorithm, selected to fulfill process requirements, to a deviation signal related to the deviation of the process value signal from the set-point signal. In the automatic mode ACS is utilized as PCS to control the final operator to minimize the deviation of the process value from the set point.
In the manual control mode an adjustable memory device called the manual memory is used to generate a constant manual control signal or MCS related to a value stored in the memory which may be adjusted by a process operator or process engineer according to process requirements.
In the direct digital control or DDC mode a digital computer is used to generate a computer control signal of CCS by applying a control algorithm, selected to fulfill process requirements, to a deviation signal related to the deviation of the process value signal from the set-point signal. In this mode CCS is used as PCS to control the final operator to minimize the deviation of the process value from the set point.
This invention still further relates to automatic process controllers for use with process installations in which an abrupt change in the process control signal is undesirable. Such abrupt changes in PCS, called bumps, may be caused by transfer between modes of operation. This invention therefore particularly relates to automatic process controllers providing automatically bumpless transfer between modes of operation.
2. Description of the Prior Art
Early automatic process control installations utilized a non-automatic transfer system requiring a "balance" mode in order to achieve bumpless transfer. In such systems an operator was required to physically adjust certain values during the balance mode in accordance with other values displayed by the controller to avoid bumps during transfer between modes. Conventional approaches provide bumpless transfer between automatic and manual modes of control in various ways. A particularly good example of this type is shown in our copending U.S. application Ser. No. 497,097 filed on even date herewith. The approach shown therein provides bumpless transfer between automatic and manual modes of operation and also allows switching between set-point sources and adjustment of set-point values and control parameters to be accomplished bumplessly. Conventional approaches do not, however, provide for bumpless transfer between the DDC mode of operation and the other modes.
The conventional approach for providing a manual memory in order to generate MCS during the manual mode of operation is shown, for example, in U.S. Pat. Nos. 3,550,014; 3,523,193; and 3,655,992 in which a capacitor in the feedback circuit of an amplifier configured as an integrator is required to store the value of MCS during the manual mode. The obvious problem inherent with the utilization of a capacitor as a memory device is drift. Such conventional devices are reliable only in the short term and will change value by leakage of current from the critical high impedance node at the input to the integrator. It is difficult to maintain the integrity of a capacitor used as a memory device and under the best of conditions a capacitor cannot be prevented from drifting. Drift in the memory results in a slowly changing MCS and PCS making the process controller unreliable in the manual mode for long periods of time.